The conductivity and permeability of certain materials at the DC level are well known. However, in a materials development research program, it is often necessary to have knowledge of a material's conductivity and permeability at certain frequencies, for example, radio frequencies. For example, in designing an effective shielding system, one needs to know the conductivity and permeability of the material under consideration under various frequencies that would be typically encountered. Generally, the higher the values of the conductivity and permeability of the material, the better it would be as a shielding element.
Prior art methods typically require large samples, which may not be available in an initial exploratory program. The traditional measure of material conductivities relies on some variant of the standard "4-probe" D.C. measurement. This determination alone is not a sufficient measure of electrical conductivity, in that the intrinsic A.C. reactance (capacitive and inductive) of any material sample is not observed by this measurement, but does have a crucial bearing on the A.C. values of these parameters. These reactive components arise quite naturally in any conductive composite as electrical manifestations of the discrete aggregate (capacitive reactance) and interconnecting filaments (resistive and inductive reactance). It is generally not possible to accurately measure these material parameters, at the frequencies of interest, using any A.C. bridge technique that makes physical contact with the test sample, because of the intrinsic parasitic reactance of the test leads.
The traditional measure of material conductivities relies on some variant of the standard "4-probe" D.C. measurement, and variants of the "Rowland Ring" for permeabilities. Indirect inductive methods are similar to Vernon et al., U.S. Pat. No. 4,922,201.
In Vernon, there exists a number of limitations and or disadvantages which include but not necessarily limited to the following: 1) The problem of coupling (the so-called "lift-off") and the associated sensitivity between the sensing probe and material to be tested; see DTIC publication ADA189823, pages 14-17. 2) The thickness of the sample can be a consideration in the method of Vernon et al. and results in the reliance on empirical relationships to deal with samples of arbitrary thickness. 3) The inductance of the probe must have increased no more than 4% above its minimum value; and there must be no frequency dependent shielding effects; see Vernon, column 4, lines 24-30 and equation 1; column 6, equation 5. 4) Vernon deals with skin depths which assume that the sample material has a permeability of free space; this feature and the empirical nature of equations 1 and 5, can limit the utility of their procedure; see equation 2 of the patent. 5) Finally, the method of Vernon et al. has the practical restriction that the probe size and frequency combination be limited such that the observed parameter be constrained between 0.8 and 6 inclusive in order to reduce the errors in subsequent computed quantities to an acceptable level; see column 5, lines 10-14 of the patent.
There is therefore a need to provide a method for measuring the conductivity and permeability of a material at various frequencies without the shortcomings of the prior art.